This invention relates to an apparatus for soldering a printed wiring board having electronic components mounted thereon.
A wave soldering apparatus needs a pump for forming a projected wave of solder. The pump supplies molten solder to a nozzle which may have various configurations and ejects it from an upper opening to generate a projected wave of solder. Naturally, at this time, the solder is heated and is in a molten state.
Various types of pumps are used in wave soldering apparatuses, such as mechanical pumps including centrifugal pumps, helical pumps, rotary blade pumps and piston pumps, and electromagnetic pumps including induction type electromagnetic pumps and conduction type electromagnetic pumps.
Each type of pump has merits and demerits. For example, mechanical pumps are highly efficient and can convert input energy to the driving motor thereof into delivery energy with little conversion loss. However, the flow of molten solder delivered from a mechanical pump is unstable and apt to fluctuate.
On the other hand, electromagnetic pumps have the merit of being able to form a stable and smooth flow of molten solder, but, in general, are inefficient and cannot convert input energy into delivery energy without a large conversion loss. Also, electromagnetic pumps have a moving magnetic field generating mechanism, namely, a magnetic circuit comprising a moving magnetic field generating coil and a core, which generates a large amount of leakage magnetic field that may cause exposure of operators to a magnetic field greater than they would receive in nature or which generates inductive current in electronic components, especially components having coils, mounted on the printed circuit board thereby damaging, imparting stress to, or at worst breaking, semiconductor components with low withstand voltages. Resistors (R), capacitors (C) and coils (L) are basic passive components comprising an electronic circuit and are used in almost all electronic circuits. Thus, coils are not special electronic components.
Linear electromagnetic pumps (LEP) directly apply an electromagnetic force to a medium to be transported to generate a thrust therein and use the thrust as a delivery force and a suction force. LEP pumps are roughly divided into two types: induction type and conduction type. In general, induction type pumps, which do not require passage of electric current through the medium to be transported, are widely used.
Induction type electromagnetic pumps include FLIP 20 type (flat linear induction pump), ALIP type (annular linear induction pump), and HIP type (helical induction pump). Each type of pump has its own peculiar constitution.
A wave soldering apparatus employing an FLIP type electromagnetic pump has been proposed in, for example, JP-A-S49-65934, JP-A-S58-122170, W097/47422 and JP-H11-104817. In the apparatuses described in JP-A-S58-122170 and JP-H11-104817, means for preventing the magnetic field generating coil from being overheated is provided. Namely, the magnetic field generating coil is installed in a position where it is easily cooled and cool air is supplied thereto with a cooling fan or the like so that the temperature of the moving magnetic field generating coil does not exceed a temperature which it can withstand. The temperature which the coil can withstand is generally determined as a temperature up to which the integrity of the insulation of the coil can be maintained.
The temperature rise in the magnetic field generating coil is attributed to copper loss caused by the current flowing in the coil itself and to iron loss caused by eddy currents generated in the iron core on which the coil is wound. In addition, the temperature rise of the magnetic field generating coil is also caused by heat transferred from the solder vessel containing the solder.
In general, a wave soldering apparatus for soldering electronic components must form a projected wave of solder having a width of at least 20 mm, a length of at least 350 mm and a height of 4-10 mm to secure sufficient time of contact between the projected wave and a printed circuit board so that the printed circuit board can be well wetted with solder. Thus, an induction type electromagnetic pump (which will be hereinafter referred to simply as xe2x80x9celectromagnetic pumpxe2x80x9d) for generating such a projected wave generally requires an input power of several kW at most.
As described in W097/47422, the flow path of the electromagnetic pump in which a moving magnetic field is applied to the solder to provide a thrust, more specifically, a delivery force and a suction force, thereto is narrow and long (for example, a few mm in width and several dozen cm in length) and has to be cleaned as necessary. Namely, it is necessary to remove the nozzle which generates the projected wave and to scrape off grime such as dross adhered thereto by inserting a long and thin rod or the like. It is also taught that the flow path may be cleaned by backwash.
Dross, which is an oxide of solder and so on, has strong adhesiveness and thus does not usually fall off by itself once adhered. Thus, when dross starts adhering in the flow path of an electromagnetic pump, it gradually accumulates and becomes large. The deposits may block the flow of solder in the flow path to cause a non-uniform flow, so that the projected wave formed above the nozzle varies in height and fluctuates in amount, resulting in lowering of the soldering quality of the work pieces such as the printed circuit boards.
Electromagnetic pumps have no movable parts such as rotating or reciprocating parts, no parts which are subjected to abrasion and no moving mechanisms such as a motor used as rotational driving means or a fluid pressure actuator used as reciprocating driving means. However, in electromagnetic pumps, pump delivery, namely, the amount of work relative to input electric power is low and energy efficiency is less than half of that of rotary pumps driven by rotary motors. Thus, a wave soldering apparatus employing an electromagnetic pump consumes a larger amount of energy as compared with a conventional wave soldering apparatus. Since most of the energy loss produced in the electromagnetic pump is radiated as heat, it is necessary to use forced cooling means such as a cooling fan in order to discharge the heat into the atmosphere.
In addition, in order to form a projected wave of the desired magnitude, it is necessary to input power of several kW at most to the electromagnetic pump to apply a very strong moving magnetic field to the solder. This causes a large leakage magnetic field from the moving magnetic field generating coil and the periphery of the core.
The leakage magnetic field is an alternating magnetic field having an amplitude which is much larger than that seen in nature. Thus, the operators must be careful in working around an electromagnetic pump. Also, when the leakage magnetic field acts on electronic components to be mounted on a printed circuit board, an inductive current which can damage or stress, or at worst break, semiconductor components for micropower, especially semiconductor components for low voltage, is generated in electronic components having coils. Particularly, when an electromagnetic pump is disposed below an opening for projecting a solder wave as in JP-AS49-65934, W097/47422 and so on, the leakage magnetic field which acts on the electronic components is the maximum.
Also, as described before, an electromagnetic pump has a narrow and long flow path which allows adhesion and deposition of grime such as dross in a short time and thus requires frequent cleaning. Additionally, in each cleaning, the nozzle must be removed and a long and thin rod must be inserted into the narrow and long path to scrape off all the grime having adhered therein. This is very difficult and inefficient work, since, as described before, dross has strong adhesion and the effect of backwashing in actual practice can hardly reach the dross having adhered and deposited in the flow path.
Moreover, there is the concern that a stray current generated by the leakage magnetic field may flow into the projected wave and then into a printed circuit board being soldered.
It is, therefore, an object of the present invention to provide a wave soldering apparatus which utilizes an electromagnetic induction pump but which has a high energy efficency, comparable to that of a conventional device using a pump driven by a rotary motor.
Another object of the present invention is to provide a soldering apparatus of the above-mentioned type, which can reduce leakage magnetic field to the minimum.
It is a further object of the present invention to provide a soldering apparatus of the above-mentioned type, in which cleaning and other maintenance of the pump and other parts can be carried out easily and securely so that the soldering cost may be reduced and the soldering quality and reliability may be improved.
It is a further object of the present invention to provide a soldering apparatus of the above-mentioned type, which can prevent stray current from flowing in the projected wave.
In accordance with the present invention there is provided a soldering apparatus for soldering a printed wiring board, comprising:
a solder vessel for containing a molten solder;
a nozzle disposed in said vessel and having a first opening at one end thereof and a second opening at the other end thereof, said first opening being disposed above the surface level of the molten solder contained in said vessel; and
an electromagnetic pump secured in said vessel and including
a tubular member defining a solder flow path therein and having an inlet and an outlet connected to said second opening of said nozzle such that the molten solder can flow from said inlet to said outlet through said path,
one or more cores, and
one or more electrically energizable coils, said tubular member, said cores and said coils being disposed below the surface level of the molten solder and arranged to generate a moving magnetic field in said path when said coils are electrically energized, so that the molten solder is fed from said inlet to said outlet and is projected from said first opening of said nozzle to form a solder wave.
In the soldering apparatus as described above, since the whole of the electromagnetic pump is immersed in the solder contained in the solder vessel, all the loss, namely thermal energy, from the electromagnetic pump is transferred to the solder and is used as energy to heat the solder without any leakage outside of the solder vessel.
Since the loss generated in the electromagnetic pump is used as energy to heat the solder, the amount of energy supplied to the solder heating means can be reduced by that amount. Namely, all the energy supplied to the electromagnetic pump and the heating means is supplied to the solder in the solder vessel and almost no energy is discharged as waste. This considerably improves the overall energy efficiency of the soldering apparatus.
The solder around the electromagnetic pump serves as a short ring (which is also referred to as xe2x80x9cshort bandxe2x80x9d and well-known in the field of power transformers) against leakage magnetic field and attenuates it, so that magnetic field leaked from the electromagnetic pump cannot leak from the solder vessel or from the surface of the solder. No magnetic field can leak out of the soldering apparatus, so that the operators, the printed circuit board and electronic parts mounted thereon are not adversely affected by leakage magnetic field.